1. Field of the Invention
The present invention relates to a method for treating materials for solidification. In particular, the present invention relates to a method for preparing materials including radioactive wastes, hazardous chemical wastes, and mixed radioactive and hazardous chemical wastes for solidification in glass or cement. The method also relates to a vitrified composition prepared by said method. The United States Government has rights in this invention pursuant to Contract No. DE-AC09-89SR18035 between the U.S. Department of Energy and Westinghouse Savannah River Company.
2. Discussion of Background
Many industrial processes generate hazardous wastes in the form of aqueous waste streams, sludge solids, aqueous supernate, incinerator ash, incinerator off gas condensate, and so forth. Waste treatment processes my themselves generate secondary hazardous wastes. For example, solids can be filtered from an aqueous waste stream by passing the stream through a perlite (PERFLO) or diatomaceous earth filter. The spent filter medium is impregnated with the materials removed from the waste stream, such as heavy metals and other hazardous substances. The spent filtration wastes must themselves be treated and stabilized before disposal. (As used herein, the term "hazardous waste"includes wastes containing substances commonly recognized as hazardous, including but not limited to, chemical wastes, radioactive wastes, mixed chemical and radioactive wastes, heavy-metal-containing wastes, and hazardous organics.)
Stabilizing hazardous wastes using currently available technology is expensive and requires enormous resources of equipment and personnel. Stabilization processes must be operated within guidelines established under the Resource Conservation and Recovery Act (RCRA), and the stabilized product must meet stringent state and federal standards. In the case of radioactive or mixed wastes, the stabilized wastes must often be stored for long periods of time waiting for decay of the radioactive components before transportation to an approved underground repository. Minimizing the waste volume is important in minimizing storage, transportation, and final disposal costs.
Incinerators are often used to destroy the hazardous constituents of solid and liquid wastes. Byproducts of incineration include bottom ash, aqueous incinerator offgas condensate (blowdown), or mixtures of ash and offgas condensate, all of which may contain residual hazardous and/or radioactive substances.
Hazardous wastes may be solidified by vitrification incorporation into a glass matrix--or cementation. In typical cementation processes, cement-forming materials are added to the waste; any water in the waste solution remains in the solidified product. Therefore, the solidified product has a larger volume than the original waste solution. Also, water, including groundwater, can leach compounds out of cement over time and cement is naturally porous, so the cement-stabilized product must be stored in leak-proof containers to prevent leaching.
Glass is the most long-term environmentally acceptable waste form. Glass is stable and extremely durable. Moreover, the hazardous species are chemically bonded in the glass structure, forming a substantially nonleachable composition. A number of vitrification processes for hazardous wastes have been described. Wheeler (U.S. Pat. No. 4,820,325) stabilizes toxic waste using a glass precursor material such as diatomaceous earth mixed with a compatible glass precursor material such as soda ash, lime (CaO) and alumina. The normally leachable toxicant becomes fixed within the glass when the mixture is vitrified. Tamara, et al. (U.S. Pat. No. 4,622,175) mix radioactive waste with water and an alkali silicate composition prepared from amorphous reactive silica (activated clay). The mixture is allowed to set in a weather-resistant container. Hayashi, et al. (U.S. Pat. No. 4,725,383) add ZnO, or a mixture of ZnO with Al.sub.2 O.sub.2 and/or CaO, to a radioactive sodium borate waste solution. The resulting mixture is dehydrated and melted to produce a vitrified solid solution. Schulz, et al. (U.S. Pat. No. 4,020,004) vitrify radioactive ferrocyanide compounds by fusion with sodium carbonate (Na.sub.2 CO.sub.3) and a mixture of basalt and B.sub.2 O.sub.3, or silica (SiO.sub.2) and lime (CaO).
The solubility of hazardous, radioactive, mixed, and heavy metal wastes during vitrification processes is often limited, resulting in hazardous residues that must in turn be treated before disposal. Retention of hazardous substances in cement is further limited by leaching. In addition, the solidified cement product may have a larger volume than the original waste solution. There is a need for an efficient, cost-effective method for treating wastes for solidification. The method should produce a stable waste composition that meets regulatory standards without adding to disposal volume.